In certain real-time computer image generation systems, objects to be displayed are represented by convex polygons which may include texture information for rendering a more realistic image. The texture information is typically stored in a plurality of two-dimensional texture maps, with each texture map containing texture information at a predetermined level of detail ("LOD") with each coarser LOD derived from a finer one by filtering as is known in the art. Further details regarding computer image generation and texturing, can be found in U.S. Pat. No. 4,727,365, U.S. Pat. No. 4,811,245 and U.S. Pat. No. 4,692,880 which are incorporated herein in their entirety by reference thereto.
Selection of the most appropriate LOD for obtaining texture for polygons representing an object requires consideration of the apparent distance of the object from a viewpoint and the angle between a vector from the viewpoint to a pixel on the polygon and the normal vector of the polygon. These two factors determine the size of the projected pixel footprint in the texture space. As the polygon moves further away from the viewpoint the footprint will decrease in size, or as it moves closer it will increase in size on the polygon. Also the angle between the view ray and the polygon normal will give a measure of how "edge on" the polygon is. As the polygon becomes more edge on in a direction the footprint will grow in size along that dimension. When moving from screen pixel to screen pixel the texture coordinate rate of change will continue to change. This non-linear relationship is why perspective mapping in necessary. The u and v texture coordinates are not linear in screen space as soon as the polygon is not perpendicular to the viewer.
Some prior art techniques had resolved to calculate an LOD value per polygon which only allowed one LOD of a map per polygon. Sometimes polygons are sub-divided (tesselated) to obtain more than one LOD per polygon. While this works well for pixels that project into polygons that are perpendicular to the viewer, texture mapping for perspective projections requires further incremental calculation techniques to generate accurate texture addresses on a per pixel basis. Such texture mapping techniques in perspective requires the calculation of texture address partial derivatives or delta values which are used to determine the appropriate LOD for texture mapping and described mathematically as: ##EQU1##
These values represent the rate of change of the U and V texture addresses with respect to one pixel change in the X or Y pixel directions on a computer display screen. Intuitively, the magnitude of each term describes the distance covered in the finest LOD level of the texture map by the projection of a pixel onto the surface of a textured polygon in the specified direction. In other words, when progressing from one pixel to the next neighboring pixel in screen X space, dU/dX is measure of how much U changes in the texture map. Another way to think of these terms is that the larger the magnitude of the term the more compressed the texture is in the specified direction. If a high frequency signal, such as a compressed texture address, is point sampled at a lower frequency, such as the pixel centers on the screen, aliasing can result. This aliasing produces undesirable visual anomalies such as scintillation or "texture swimming", where the texture pattern appears to float around on the top of a polygon when in motion. By pre-filtering the texture images into multiple LODs the unwanted visual artifacts can be avoided. The above terms are used to select the texture image which has the appropriate amount of filtering applied on a pixel-by-pixel basis. The term with the largest rate of change should dominate the LOD selection criteria, otherwise the sampled texture in that direction may scintillate. This approach generally minimizes visual artifacts at the expense of blurring the resulting image in other directions. A simple maximum of all four gradient terms however, is not sufficient because the magnitude of the terms can change as the polygon rotates and effectively select a different LOD for the same pixel, depending on the orientation of the polygon. One approach for reducing this problem has been proposed as set forth by Paul Heckbert's Master's Thesis, University of California, Berkeley, June 1989 which sets forth an LOD calculation in accordance with equation (1) as follows: ##EQU2##
Techniques have been implemented to approximate such texture address partial derivatives, however, they have proven to be very computer resource intensive, requiring many floating point operations which are often difficult and impractical to implement in real-time, 3-D computer graphics applications.
Thus, it would be highly desirable to provide a method for generating instantaneous texture address values used to calculate an LOD on a per pixel basis, in real-time, that requires minimum operations and results in a visually effective texture mapped perspective object.